The present invention relates to a particulate material comprising an alloy or composite. The particulate material is especially suitable for use as a feed material in the injection moulding or casting of thixotropic alloys. As used herein, the terms "composite" or "alloy composite" include an alloy matrix having ceramic reinforcement, and includes metal matrix composites.
The semi-solid processing of alloys and composites is an area of technology in which much interest is presently being shown. Such processing generally requires the formation of a thixotropic alloy which is subsequently processed. Thixotropic alloys are produced when solid particles of a metal or alloy are homogeneously suspended in a liquid phase of molten metal. The semi-solid mass thus produced has thixotropic rheology.
Thixotropic alloys may be processed to produce metal articles by injection moulding.
A number of processes to produce thixotropic alloys have been proposed. U.S. Pat. Nos. 4,694,881 and 4,694,882 both assigned to the Dow Chemical Corp., the entire contents of which are herein incorporated by reference, describe processes for producing thixotropic alloys which comprise feeding solid particles of a metal alloy from a hopper into an extruder, such as a screw extruder. In U.S. Pat. No. 4,694,881, the solid particles are heated in the extruder to a temperature above the liquidus temperature of the alloy. The molten mass thus obtained is subsequently cooled to a temperature between the solidus and liquidus temperatures and subjected to shearing to break the dendritic structure that would otherwise form. The resulting liquid-solid composition of a thixotropic alloy is injected into a mould to form a moulded product.
U.S. Pat. No. 4,694,882 describes a similar process, except that the feed alloy particles are heated to a temperature between the solidus and liquidus temperatures, without complete melting of the feed metal particles taking place.
Both of the above processes utilise feed particles or chips of a convenient size for handling. The patents especially describe the use of chips having an irregular shape. The size of the particles used is described as not being critical to the invention, although relatively small particle sizes are preferred because of heat transfer and handling requirements.
Experiments carried out by the present applicant have shown that the particles used in the process described in U.S. Pat. Nos. 4,694,881 and 4,694,882 are prone to block the hopper and seize the screw extruder. Further, the particles do not exhibit good packing characteristics which can cause difficulty in achieving sufficient heat transfer rates to cause the partial melting of the metal particles and also render control over the temperature more difficult.